Fair Value (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Long-term Debt	$ 1,394.4	$ 1,393.6
Subsidiaries [Member]
Long-term Debt	574.0	574.1
Cost [Member]
Total Assets	6.1	6.4
Cost [Member] \| Subsidiaries [Member]
Total Assets	6.1	6.4
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.2	8.9
Total Assets	9.2	8.9
Fair Value [Member] \| Subsidiaries [Member]
Total Assets	9.2	8.9
Money Market Funds [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.3	0.3
Money Market Funds [Member] \| Cost [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Cost	0.3	0.3
Money Market Funds [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Money Market Funds [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Equity Securities [Member] \| Cost [Member]
Total Master Trust Assets, Cost	2.0	2.1
Equity Securities [Member] \| Cost [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Cost	2.0	2.1
Equity Securities [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	5.0	4.5
Equity Securities [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	5.0	4.5
Debt Securities [Member] \| Cost [Member]
Total Master Trust Assets, Cost	3.8	4.0
Debt Securities [Member] \| Cost [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Cost	3.8	4.0
Debt Securities [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	3.9	4.1
Debt Securities [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	3.9	4.1
Debt [Member] \| Cost [Member]
Long-term Debt		1,393.6
Debt [Member] \| Cost [Member] \| Subsidiaries [Member]
Long-term Debt	574.0	574.1
Debt [Member] \| Fair Value [Member]
Debt, Fair Value	1,530.1	1,571.6
Debt [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Debt, Fair Value	$ 632.5	$ 656.0

The total of assets that are measured on a recurring basis.

No definition available.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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- Definition

Includes other investments and securities that are not at fair value as of the balance sheet date and those that are not accounted for under the equity method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 942
-SubTopic 210
-Section S99
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